1. Field of the Invention
The present invention relates to spacecraft and payload control systems, and in particular to a system and method for directing a spacecraft payload at a specified target location.
2. Description of the Related Art
Satellites, particularly those in geosynchronous or geostationary orbits, are an economical means for transmitting information over vast distances. These applications typically require that the sensitive axis of the payload (which can include a camera or communication antenna) be directed to point to a precise location on the surface of the Earth.
U.S. Pat. No. 4,688,091, issued on Aug. 18, 1987 to Kamel et al. discloses a system for directing a spacecraft payload to a precise location on the Earth""s surface. While this system allows for precise payload pointing, it requires that the payload itself be steered. This technique is inapplicable to payloads that cannot be steered independently from the satellite body, and is an expensive and complex solution, even for payloads that allow for independent steering. Further, the system disclosed in this reference relies upon data sensed by the payload itself (i.e. a camera) for precise pointing. This solution is inapplicable to payloads that cannot provide such data, or do not have the throughput and bandwidth to provide such data at a latency and rate as required.
What is needed is a system and method for accurate payload steering that is effective with a wide variety of payloads, and which can be inexpensively implemented according to the pointing accuracy requirements for each specific payload. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method, apparatus, and article of manufacture for directing a payload coupled to a spacecraft substantially at a target location.
The method comprises the steps of determining spacecraft orbit parameters, spacecraft bus attitude parameters and spacecraft payload attitude parameters, and directing the payload at the target location by applying a spacecraft bus steering law determined at least in part from the orbit and bus/payload attitude parameters. The apparatus comprises a navigation system for determining spacecraft orbit parameters and spacecraft attitude parameters, and for applying a spacecraft bus steering law to direct the payload substantially at the target location. The spacecraft bus steering law is determined at least in part from the determined spacecraft orbital parameters, the determined spacecraft attitude, and determined payload attitude parameters. In one embodiment, the apparatus includes a payload attitude control system for directing the payload relative to the spacecraft bus. The article of manufacture comprises a program storage device tangibly embodying a series of instructions for performing the above method steps.
The foregoing invention uses xe2x80x9csmart steeringxe2x80x9d to steer the spacecraft body (i.e. spacecraft bus) such that the pointing registration (alignment of the payload line-of-sight relative to the earth""s surface) error of a directionally sensitive payload, such as a camera, due to payload thermal deformation and spacecraft attitude error can be compensated. The payload can be mechanically steerable, electronically steerable or unsteerable with respect to the spacecraft bus. The foregoing invention also allows the dynamic range of spacecraft payload pointing registration error due to spacecraft orbit error to be minimized. Further, the payload steering algorithm for compensating payload pointing registration error (if implemented) can be simplified.
These smart steering techniques can be used with tight orbit control. This allows precise payload pointing registration to be achieved without implementing the payload steering correction algorithm.